Composite Mandrel For Autoclave Curing Applications

ABSTRACT

A composite mandrel includes a generally elongated mandrel body comprising a resilient mandrel core and an elastomeric mandrel outer layer disposed outside the mandrel core. A method for fabricating a contoured stiffened composite panel is also disclosed.

TECHNICAL FIELD OF THE INVENTION

The disclosure relates to mandrels for forming cavities in composite materials. More particularly, the disclosure relates to a composite mandrel which is suitable for autoclave curing applications in the formation of cavities in composite materials.

BACKGROUND OF THE INVENTION

When composite materials are molded into shapes with cavities, such as hat stringers, for example, there may be a need for some type of tooling that can apply pressure from the cavity outward during the curing step and can be extracted from the cavity after curing. The existing tooling used for this purpose may include without limitation inflatable rubber mandrels; solid mandrels such as metal, rubber or composite mandrels; or dissolvable mandrels. However, the inflatable rubber mandrels may be prone to leaking, which may lead to widespread porosity in the resulting composite laminate. The solid rubber mandrel may result in a cavity with a distorted cross-sectional shape or exert an uneven pressure on the composite laminate and may be too heavy for fabrication of large parts. The solid metal or composite mandrels may not have sufficient flexibility to be removed from parts having any degree of curvature or complexity. The dissolvable mandrels may be expensive to make and difficult to remove from large parts. Existing mandrel designs may not accommodate the dimensional changes of the composite part which occurs during application of heat to the surrounding tooling and part materials at the curing step. This can cause undesirable part material movement resulting in such distortions as waviness, wrinkling and/or bridging in the composite material.

Therefore, a mandrel is needed which is suitable for curing applications in the formation of cavities in composite materials and overcomes some or all of the limitations of conventional composite mandrels.

SUMMARY OF THE INVENTION

The disclosure is generally directed to a composite mandrel. An illustrative embodiment of the composite mandrel includes a generally elongated mandrel body comprising a resilient mandrel core and an elastomeric mandrel outer layer disposed outside the mandrel core. The mandrel may combine the desired characteristics of foam and rubber to produce a manufacturing aid for airplane stringers or other similar open cavity parts made from fiber/resin composite materials. The manufacturing aid which is embodied in the composite mandrel may be less costly, more durable and less prone to failures than current inflatable bladder technologies.

BRIEF DESCRIPTION OF THE ILLUSTRATIONS

FIG. 1 is a top view of an illustrative embodiment of the composite mandrel.

FIG. 2 is a cross-sectional view, taken along section lines 2-2 in FIG. 1, of the composite mandrel.

FIG. 3 is a cross-sectional view of an alternative illustrative embodiment of the composite mandrel.

FIG. 4 is an exploded top view of a composite assembly, more particularly illustrating insertion of multiple composite mandrels into respective stiffening elements in the composite assembly preparatory to curing of the composite assembly.

FIG. 5 is a cross-sectional view, taken along section lines 5-5 in FIG. 4, of the composite assembly.

FIG. 6 is a top view of the composite assembly, with the composite mandrels inserted in the respective stiffening elements of the assembly.

FIG. 7 is a top view of the composite assembly, contained in vacuum bagging preparatory to curing of the assembly.

FIG. 8 is an exploded top view of the composite assembly, more particularly illustrating removal of the composite mandrels from the respective stiffening elements in the composite assembly after curing of the composite assembly.

FIG. 9 is a flow diagram which illustrates an illustrative method for fabricating a contoured stiffened composite panel.

FIG. 10 is a flow diagram of an aircraft production and service methodology.

FIG. 11 is a block diagram of an aircraft.

DETAILED DESCRIPTION

Referring initially to FIGS. 1 and 2, an illustrative embodiment of the composite mandrel is generally indicated by reference numeral 1. The composite mandrel 1 may be used to fill a cavity (not shown) in an airplane stringer or other open-cavity part (not shown) made from fiber/resin composite materials to prevent collapse of the cavity during curing of the composite materials. The composite mandrel 1 may be less costly, more durable and more effective and reliable than current inflatable bladder mandrel technologies.

The composite mandrel 1 includes a generally elongated mandrel body 7 having a mandrel core 2 which is a resilient material and a mandrel outer layer 10 which is disposed outside the mandrel core 2, as shown in FIG. 2, and is an elastomeric material. In some embodiments, the mandrel core 2 is foam or other such material which incorporates open space and/or air pockets to prevent bulk modulus behavior during thermal expansion and the mandrel outer layer 10 may be an elastomeric material such as elastic rubber, for example and without limitation. The mandrel core 2 and the mandrel outer layer 10 may be generally coextensive with the mandrel body 7.

The mandrel core 2 and the mandrel outer layer 10 may have any cross-sectional shape depending on the particular use requirements of the composite mandrel 1. In some applications, for example, each of multiple composite mandrels 1 may be suitably configured to fill respective stiffening elements (such as stringers) 27 during the curing and/or cocuring of a composite panel assembly 24, as shown in FIGS. 4-8 and will be hereinafter described. As shown in FIG. 2, in some embodiments of the composite mandrel 1, the mandrel body 7 may have a generally triangular cross-sectional shape. Accordingly, the mandrel core 2 has a generally flat or planar core base 3 with lateral core edges 6. Core sides 4 angle from the respective core edges 6. A core apex 5, which may be rounded, extends between the core sides 4. The shape of the mandrel outer layer 10 may generally correspond to that of the mandrel core 2, defining a mandrel base 11 which extends adjacent to the core base 3; a pair of mandrel sides 12 which extend adjacent to the respective core sides 4; a mandrel apex 13 which may be rounded and is disposed adjacent to the core apex 5; and mandrel edges 14 which correspond positionally to the respective core edges 6 of the mandrel core 2.

As shown in FIG. 3, in some embodiments of the composite mandrel 1 a, the mandrel body 7 a may have a generally trapezoidal shape. Accordingly, the mandrel core 2 a has a generally flat or planar core base 3; a pair of core sides 4 which angle from the core base 3; and a generally flat or planar mandrel core top 8 which extends between the core sides 4. The mandrel outer layer 10 a defines a mandrel base 11 which extends adjacent to the core base 3; a pair of mandrel sides 12 which extend adjacent to the respective core sides 4; a generally flat or planar mandrel top surface 16 which is disposed adjacent to the mandrel core top 8; and mandrel edges 14 which correspond to the respective core edges 6 of the mandrel core 2 a.

Referring next to FIGS. 4-8, in typical application, multiple composite mandrels 1 are inserted in respective stiffening elements 27 provided in a stiffening layer 26 of a composite panel assembly 24 during curing of the composite panel assembly 24. The composite panel assembly 24 will ultimately form an airplane stringer (not shown); however, it will be appreciated by those skilled in the art that the composite mandrels 1 can be adapted to fill cavities in any other type of open-cavity or closed-cavity composite material part made from fiber/resin composite materials during curing of the composite material part. The composite mandrels 1 can be adapted to fill cavities having a constant cross-sectional shape or a cross-sectional shape which varies along the length of the composite material, such as cavities which taper or curve along the length of the cavity, for example and without limitation.

As illustrated in FIG. 5, in an embodiment of fabrication of the composite panel assembly 24, a base composite layer 25 may initially be placed on a tooling surface 20 of OML tooling or IML tooling, for example and without limitation. The tooling surface 20 may have a generally concave contour, as shown. Alternatively, the tooling surface 20 may have a generally planar or convex contour, depending on the particular application. The stiffening layer 26 may be placed on the base composite layer 25. The stiffening elements 27 may be shaped in the stiffening layer 26 and extend along the longitudinal axis of the tooling surface 20 in generally parallel relationship with respect to each other, as shown in FIG. 4, and in generally perpendicular relationship with respect to the concave contour of the tooling surface 20. Alternatively, the stiffening elements 27 may be separate or discrete units. As further shown in FIG. 5, each stiffening element 27 has a stiffening element cavity 28. In some embodiments, the stiffening elements 27 may be oriented in orientations other than along the longitudinal axis of the tooling surface 20 and may converge or diverge, for example and without limitation.

As shown in FIGS. 4 and 6, multiple composite mandrels 1 may be inserted into the stiffening element cavitys 28 of the respective stiffening elements 27. The elastomeric mandrel outer layer 10 of each composite mandrel 1 allows for a proper fit of the composite mandrel 1 into the stiffening element cavity 28 of each stiffening element 27 and conforms to pad-ups and ramps. As shown in FIG. 7, the composite panel assembly 24 may then be enclosed in vacuum bagging 30 and cured by autoclaving. During the curing process, the composite mandrels 1 maintain the shape and prevent collapse of the respective stiffening elements 27 as the composite material of the base composite layer 25 and the stiffening layer 26 hardens.

After curing, the composite panel assembly 24 is removed from the vacuum bagging 30. The composite mandrels 1 may be removed from the stiffening element cavitys 28 of the respective stiffening elements 27, as shown in FIG. 8. During removal, the elastomeric mandrel outer layer 10 of each composite mandrel 1 may easily be deformed; this reduces the effort required for removal. The cured composite panel assembly 24 may then be processed to complete fabrication of the airplane assembly (not shown) or other composite part, according to the knowledge of those skilled in the art.

It will be appreciated by those skilled in the art that the resilient mandrel core 2 of the composite mandrel 1 enhances the structural and compressive characteristics of the composite mandrel 1 relative to the designs of conventional mandrels. This structural and compressive support may be necessary to maintain the shape of the stringer or other composite part during automated composite fiber placement as well as autoclave curing. Since the outer mandrel layer 10 may be a constant thickness, it may expand uniformly during curing, thus avoiding the problems associated with uneven expansion of a solid rubber material. The cross-sectional area and type of foam used for the mandrel core 2 may be engineered to impart compression compliance under autoclave pressure, thus offsetting the combined thermal expansion behavior of the foam and rubber.

Referring next to FIG. 9 of the drawings, a flow diagram 900 which illustrates an illustrative method for fabricating a contoured stiffened composite panel is shown. In block 902, a tooling surface, such as the tooling surface 20 which was heretofore described with respect to FIG. 5, for example and without limitation, is provided. The tooling surface may have a concave, planar, convex or alternative contour. In block 904, a base composite layer is laminated on the tooling surface. In block 906, open-section stiffening elements are positioned on the base composite layer. In block 908, composite mandrels are provided. Each composite mandrel includes a resilient mandrel core and an elastomeric mandrel outer layer disposed outside the resilient mandrel core. In block 910, composite mandrels are inserted in the respective stiffening elements. In block 912, the composite panel and stiffening elements are sealed in vacuum bagging. In block 914, the composite panel and the stiffening elements are cured. An autoclave may be used during curing. In block 916, the composite mandrels are removed from the stiffening elements.

Referring next to FIGS. 10 and 11, embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method 78 as shown in FIG. 10 and an aircraft 94 as shown in FIG. 11. During pre-production, exemplary method 78 may include specification and design 80 of the aircraft 94 and material procurement 82. During production, component and subassembly manufacturing 84 and system integration 86 of the aircraft 94 takes place. Thereafter, the aircraft 94 may go through certification and delivery 88 in order to be placed in service 90. While in service by a customer, the aircraft 94 may be scheduled for routine maintenance and service 92 (which may also include modification, reconfiguration, refurbishment, and so on).

Each of the processes of method 78 may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on.

As shown in FIG. 11, the aircraft 94 produced by exemplary method 78 may include an airframe 98 with a plurality of systems 96 and an interior 100. Examples of high-level systems 96 include one or more of a propulsion system 102, an electrical system 104, a hydraulic system 106, and an environmental system 108. Any number of other systems may be included. Although an aerospace example is shown, the principles of the invention may be applied to other industries, such as the automotive industry.

The apparatus embodied herein may be employed during any one or more of the stages of the production and service method 78. For example, components or subassemblies corresponding to production process 84 may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft 94 is in service. Also, one or more apparatus embodiments may be utilized during the production stages 84 and 86, for example, by substantially expediting assembly of or reducing the cost of an aircraft 94. Similarly, one or more apparatus embodiments may be utilized while the aircraft 94 is in service, for example and without limitation, to maintenance and service 92.

Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art. 

1. A composite mandrel, comprising: a generally elongated mandrel body comprising: a resilient mandrel core; and an elastomeric mandrel outer layer disposed outside said mandrel core.
 2. The composite mandrel of claim 1 wherein said mandrel core comprises foam.
 3. The composite mandrel of claim 1 wherein said mandrel outer layer comprises an elastomeric material.
 4. The composite mandrel of claim 1 wherein said mandrel body has a generally triangular cross-section.
 5. The composite mandrel of claim 4 wherein said mandrel core comprises a core base, a pair of core sides extending from said core base and a core apex extending between said pair of core sides.
 6. The composite mandrel of claim 5 wherein said core apex of said mandrel core is rounded.
 7. The composite mandrel of claim 5 wherein said mandrel outer layer comprises a mandrel base disposed adjacent to said core base of said mandrel core; a pair of mandrel sides disposed adjacent to said core sides, respectively, of said mandrel core; and a mandrel apex disposed adjacent to said core apex of said mandrel core.
 8. The composite mandrel of claim 7 wherein said mandrel apex of said mandrel outer layer is rounded.
 9. A composite mandrel for fabricating an aircraft part, comprising: a generally elongated mandrel body having a generally trapezoidal cross-section and comprising: a resilient mandrel core; and an elastomeric mandrel outer layer disposed outside said mandrel core.
 10. The composite mandrel of claim 9 wherein said mandrel core comprises foam.
 11. The composite mandrel of claim 9 wherein said mandrel outer layer comprises an elastomeric material.
 12. The composite mandrel of claim 9 wherein said mandrel core comprises a core base, a pair of core sides extending from said core base and a generally planar core top extending between said pair of core sides.
 13. The composite mandrel of claim 12 wherein said mandrel outer layer comprises a mandrel base disposed adjacent to said core base of said mandrel core; a pair of mandrel sides disposed adjacent to said core sides, respectively, of said mandrel core; and a mandrel top surface disposed adjacent to said core top of said mandrel core.
 14. A method for fabricating a contoured stiffened composite panel for an aircraft structure, comprising: providing a tooling surface; placing a base composite layer on said tooling surface; placing at least one stiffening element having a stiffening element cavity on said base composite layer; providing at least one composite mandrel having a resilient mandrel core and an elastomeric mandrel outer layer disposed outside said resilient mandrel core; inserting said at least one composite mandrel in said stiffening element cavity of said at least one stiffening element; curing said base composite layer and said at least one stiffening element; and removing said at least one composite mandrel from said stiffening element cavity of said at least one stiffening element.
 15. The method of claim 14 wherein said providing a tooling surface comprises providing a tooling surface having a generally concave contour.
 16. The method of claim 14 wherein said curing said base composite layer and said at least one stiffening element comprises providing vacuum bagging, placing said base composite layer and said at least one stiffening element in said vacuum bagging and subjecting said vacuum bagging to autoclave conditions.
 17. The method of claim 14 wherein said providing at least one composite mandrel comprises providing at least one composite mandrel having a generally triangular cross-section.
 18. The method of claim 14 wherein said providing at least one composite mandrel comprises providing at least one composite mandrel having a generally trapezoidal cross-section.
 19. The method of claim 14 wherein said providing at least one composite mandrel having a resilient mandrel core comprises providing at least one composite mandrel having a foam mandrel core.
 20. The method of claim 14 wherein said providing at least one composite mandrel having an elastomeric mandrel outer layer comprises providing at least one composite mandrel having an elastic rubber mandrel outer layer.
 21. A composite mandrel for fabricating an aircraft part, comprising: a generally elongated mandrel body having a generally trapezoidal cross-section and comprising: a resilient foam mandrel core having a core base, a pair of core sides extending from said core base and a generally planar core top extending between said pair of core sides; and an elastic rubber mandrel outer layer disposed outside said mandrel core and having a mandrel base disposed adjacent to said core base of said mandrel core; a pair of mandrel sides disposed adjacent to said core sides, respectively, of said mandrel core; and a mandrel top surface disposed adjacent to said core top of said mandrel core.
 22. A method for fabricating a contoured stiffened composite panel for an aircraft structure, comprising: providing a tooling surface having a generally concave contour; placing a base composite layer on said tooling surface; placing at least one stiffening element having a stiffening element cavity on said base composite layer; providing at least one composite mandrel having a generally triangular or trapezoidal cross-section and including a resilient foam mandrel core and an elastic rubber mandrel outer layer disposed outside said resilient mandrel core; inserting said at least one composite mandrel in said stiffening element cavity of said at least one stiffening element; curing said base composite layer and said at least one stiffening element by providing vacuum bagging, placing said base composite layer and said at least one stiffening element in said vacuum bagging and subjecting said vacuum bagging to autoclave conditions; and removing said at least one composite mandrel from said stiffening element cavity of said at least one stiffening element. 